1. Field of the Invention
The present invention relates to a lithium (Li) secondary cell, and more particularly, to an organic electrolyte for a lithium secondary cell, which can improve high-temperature characteristic, self-discharge characteristics and charging/discharging cycle characteristics as well as cell capacity, withstand low temperatures, and to a lithium secondary cell employing the same.
2. Description of the Related Art
Recently, electronic products have become small, thin and light, and portable electronic products such as camcorders, lap-top computers and cellular phones have become widespread, placing greater demands on the performance of secondary cells used as power sources. As a cell capable of satisfying such demands, a Li secondary rechargeable cell having a small size and a light weight has been focused on and rapidly developed to replace well-known lead-acid cells, nickel-cadmium cells, etc.
A Li rechargeable cell comprises a positive electrode and a negative electrode, each formed of materials capable of allowing intercalation and deintercalation of Li ions, and an organic electrolyte or polymer electrolyte, in which Li ions can move, filled between the positive electrode and the negative electrode. Here, electrical energy is generated by oxidation and reduction reactions when the Li ions are intercalated/deintercalated in the positive electrode and negative electrode.
The positive electrode of the Li rechargeable cell can be formed of a composite oxide of a transition metal and Li, such as lithium cobalt oxide (LiCoO.sub.2), lithium nickel oxide (LiNiO.sub.2) or lithium manganese oxide (LiMnO.sub.2), which has an electric potential higher than that of a Li/Li.sup.+ electrode by as much as 3.about.4.5V, and allows intercalation/deintercalation of Li ions.
The negative electrode is formed of Li metal or its alloy capable of reversibly accepting or providing Li ions without changing its structure and electrical characteristics, or a carbonic material having similar chemical potential to that of Li metal during intercalation/deintercalation of Li ions.
A cell adopting Li metal or its alloy as an active material is called a Li metal cell, and a cell adopting a carbonic material as an active material is called a lithium ion cell. In the Li metal cell using the Li metal or its alloy as a negative electrode, the volume of Li metal changes during charging/discharging, and Li is locally educed on the surface of the Li metal, shorting the cell. Accordingly, the cell becomes unstable and wears out quickly. Thus, it is difficult to introduce the cell to a market. In order to solve these problems, a Li ion cell adopting a carbonic material as a negative electrode active material has been developed. In the Li ion cell, Li ions merely move during charging/discharging, without changing the negative electrode active material, so that the life span and stability are much better than in a Li metal cell.
Also, a Li polymer cell is another type of Li secondary cell, adopting a solid polymer electrolyte . The Li polymer cell can be either a solid Li polymer cell containing solid-type electrolyte without any organic electrolyte, or a gel-type Li polymer cell adopting a gel-type polymer electrolyte containing an organic electrolyte impregnated into a polymer, according to the type of polymer electrolyte. Also, the Li polymer cell can be either a Li ion polymer cell or a Li metal polymer cell, according to the negative electrode active material as described above.
The organic electrolyte is an important factor in determining the performance of the Li polymer cell as well as the Li ion cell. The organic electrolyte, is an ion conductive material obtained by dissolving lithium salts in an organic solvent, and must have excellent Li ion conductivity, and chemical and electrochemical stability with the electrode. Also, its usable temperature range must be wide and it must be cheap to manufacture. Thus, it is preferable to use an organic solvent having high ion conductivity and dielectric constant and low viscosity.
However, because no single organic solvent completely satisfies the above conditions, an organic solvent mixture containing an organic solvent having a high dielectric constant and an organic solvent having a low viscosity is used, e.g., a solvent mixture of carbonic esters containing propylenecarbonate and diethylcarbonate or a solvent mixture containing ethylenecarbonate, dimethylcarbonate and diethylcarbonate.
Such organic solvent mixtures increase the mobility of lithium ions, so ion conductivity is markedly improved and the initial capacity of a cell is increased. However, repeating cycles reduces capacity, due to an oxidation reaction of the electrolyte with a negative electrode active material. Also, due to freezing of the organic solvent at a low temperature, the mobility of Li ions decreases. Accordingly, there is high possibility that the ion conductivity suddenly drops.
Japanese Patent Application Publication No. Heisei 7-169504 discloses an organic electrolyte obtained by adding a third solvent containing methylpropyonate and ethylpropyonate having a very low viscosity to a conventional two-component organic solvent containing a solvent having a high dielectric constant and a low-viscosity solvent. However, even if such an organic electrolyte improves the low-temperature discharge characteristics, the room temperature life span characteristics are deteriorated, and a reaction product of a spontaneous reaction with a collector contaminates the electrolyte, causing poor cell characteristics.
Also, Li salts in an organic electrolyte for a Li secondary cell include LiClO.sub.4, LiPF.sub.6, LiCF.sub.3 SO.sub.3, or LiN(CF.sub.3 SO.sub.2).sub.2. However, such currently used Li salts have poor thermal stability and low ion conductivity. In particular, because LiPF.sub.6 is very sensitive to moisture despite its excellent ion conductivity, the electrolyte itself decomposes easily.